The present invention relates to a control method and, more particularly, to a control method capable of preventing a motor from damage.
As shown in FIG. 1, a conventional motor comprises a stator 10 and a rotor 12. A sensor 16 is disposed on the stator 10, and four north poles (hereinafter N pole) and four south poles (hereinafter S pole) are alternatively on the rotor 12. When the sensor 16 detects an N pole or an S pole of the rotor 12, current direction in the stator coil (not shown) is changed to change the polarity of the iron core of the stator 10. When an N pole 121 is detected by the sensor 16, a high level signal is output and the polarity of iron core 101 of the stator 10 is changed to north. Thereby, rotation of the rotor 12 is maintained by the repulsion between the N pole 121 and the iron core 101. When an S pole is detected by the sensor 16, a low level signal is output and the polarity of iron core 101 is changed to south, thereby maintaining the rotation of the rotor 12. The output signal Snom of the sensor 16 is represented in FIG. 3a. 
If the boundary between the N pole 121 and an S pole 122 is positioned near the senor 16 when the rotor 12 stops, the sensor 16 detects the N pole 121 first and the polarity of the iron core 101 is changed to north and the rotor 12 rotates clockwise when the motor restarts. As shown in FIG. 2, the rotor 12 rotates continuously. When the S pole 122 is detected by the sensor 16, the polarity of the iron core 101 is changed to south. Thereby, the rotor 122 is rotated counterclockwise by the repulsion between the S pole 122 of the rotor 12 and the iron core 101. In turn, the N pole 121 is again detected by the sensor 16 and the clockwise rotation resumes. Such repeated clockwise and counterclockwise rotations cause the rotor 12 to shake. The output signal Sabnom generated when the sensor 16 shakes is shown in FIG. 3b. The shake of rotor 12 can cause motor damage due to current overload.
The number of boundaries between the N and S poles depends on the number of the N poles or S poles. In FIGS. 1 and 2, the rotor 12 is provided with 8 poles (including N and S poles), therefore 8 boundaries are provided. The number of boundaries of a rotor has direct effect on the amount of shaking.